Optical micromanipulation has expanded into a substantial field that continues to have major impact upon the biological and colloidal sciences, see Dholakia, K. et al, Optical micromanipulation. Chem. Soc. Rev. 37, 42-55 (2008). A particular theme that has emerged is the use of novel light fields and extended arrays of traps: these may be grouped under the particular title of “optical potential energy landscapes”. The ability to influence the movement of colloidal and biological objects in such landscapes is of importance in observing phase transitions and to initiate optical sorting. Such microparticles respond to the gradient and scattering forces exerted and follow trajectories not commensurate with the flow direction of the liquid in which the particles are provided.